Membrane
bioreactor (MBR) is the
combination of a membrane process like microfiltration or ultrafiltration with a
suspended growth bioreactor, and is
now widely used for municipal and industrial wastewater treatment
with plant sizes up to 80,000 population equivalent (i.e. 48
million liters per day)/
Overview
When used with domestic wastewater, MBR processes can produce
effluent of high quality enough to be discharged to coastal, surface or
brackish waterways or to be reclaimed for urban irrigation. Other advantages of
MBRs over conventional processes include small footprint, easy retrofit and
upgrade of old wastewater treatment plants
It is possible to operate MBR
processes at higher mixed liquor suspended solids (MLSS) concentrations compared to conventional settlement
separation systems, thus reducing the reactor volume to achieve the same
loading rate.
Two MBR configurations exist:
internal/submerged, where the membranes are immersed in and integral to the
biological reactor; and external/sidestream, where membranes are a separate
unit process requiring an intermediate pumping step.
Recent technical innovation and
significant membrane cost reduction have enabled MBRs to become an established
process option to treat wastewaters. As a result, the MBR process has now become an attractive
option for the treatment and reuse of industrial and municipal wastewaters, as
evidenced by their constantly rising numbers and capacity. The current MBR
market has been estimated to value around US$216 million in 2006 and to rise to
US$363 million by 2010.
Membrane bioreactors can be used to
reduce the footprint of an activated sludge sewage treatment system by removing
some of the liquid component of the mixed liquor. This leaves a concentrated
waste product that is then treated using the activated
sludge process.
MBR history and basic operating
parameters
The MBR process was introduced by
the late 1960s, as soon as commercial scale ultrafiltration (UF) and microfiltration (MF) membranes were available. The original process was
introduced by Dorr-Oliver Inc. and combined the use of an activated
sludge bioreactor with a crossflow
membrane filtration loop. The flat sheet membranes used in this process were
polymeric and featured pore sizes ranging from 0.003 to 0.01 μm. Although the
idea of replacing the settling
tank of the conventional activated
sludge process was attractive, it was difficult to justify the use of such a
process because of the high cost of membranes, low economic value of the
product (tertiary effluent) and the potential rapid loss of performance due to
membrane fouling. As a result, the focus was on the attainment of high fluxes,
and it was therefore necessary to pump the MLSS at high crossflow velocity at
significant energy penalty (of the order 10 kWh/m3 product) to
reduce fouling. Because of the poor economics of the first generation MBRs,
they only found applications in niche areas with special needs, such as
isolated trailer parks or ski resorts.
The breakthrough for the MBR came in
1989 with Yamamoto and co-workers idea of submerging the membranes in the
bioreactor. Until then, MBRs were designed with the separation device located
external to the reactor (sidestream MBR) and relied on high transmembrane
pressure (TMP) to maintain filtration. With the membrane directly immersed in
the bioreactor, submerged MBR systems are usually preferred to sidestream
configuration, especially for domestic wastewater treatment. The submerged
configuration relies on coarse bubble aeration
to produce mixing and limit fouling. The energy demand of the submerged system
can be up to 2 orders of magnitude lower than that of the sidestream systems
and submerged systems operate at a lower flux, demanding more membrane area. In
submerged configurations, aeration is considered as one of the major parameters
in process performance both hydraulic and biological. Aeration maintains solids
in suspension, scours the membrane surface and provides oxygen to the biomass,
leading to a better biodegradability and cell synthesis.
The other key steps in the recent
MBR development were the acceptance of modest fluxes (25 percent or less of
those in the first generation), and the idea to use two-phase bubbly flow to
control fouling. The lower operating cost obtained with the submerged
configuration along with the steady decrease in the membrane cost encouraged an
exponential increase in MBR plant installations from the mid 90s. Since then,
further improvements in the MBR design and operation have been introduced and
incorporated into larger plants. While early MBRs were operated at solid
retention times (SRT) as high as 100 days with MLSS up to 30 g/L, the recent
trend is to apply lower solid retention times (around 10–20 days), resulting in
more manageable MLSS levels (10 to 15 g/L). Thanks to these new operating
conditions, the oxygen transfer and the pumping cost in the MBR have tended to
decrease and overall maintenance has been simplified. There is now a range of
MBR systems commercially available, most of which use submerged membranes
although some external modules are available; these external systems also use
two-phase flow for fouling control. Typical hydraulic retention times (HRT)
range between 3 and 10 hours. In terms of membrane configurations, mainly
hollow fibre and flat sheet membranes are applied for MBR applications.
Despite the more favourable energy
usage of submerged membranes, there continued to be a market for the side
stream configuration, particularly in industrial applications. For ease of
maintenance the side stream configuration can be installed on a lower level in
a plant building. Membrane replacement can be undertaken without specialised
lifting equipment. As a result research continued with the side stream
configuration, during which time it was found that full scale plants could be
operated with higher fluxes. This has culminated in recent years with the
development of low energy systems which incorporate more sophisticated control
of the operating parameters coupled with periodic back washes, which enable
sustainable operation at energy usage as low as 0.3 kWh/m3 of
product.
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